Among digital modulation methods, frequency shift keying or FSK, amplitude shift keying and phase shift keying, as well as combinations of these methods, are broadly utilized for the digital modulation of a carrier signal.
One option for subjecting a carrier signal to frequency shift keying consists of directly modulating the carrier signal when it is generated in the oscillator.
In this case, the frequency shift keying of the oscillator is usually achieved by connecting or disconnecting small, frequency-determining capacitors to a resonant circuit formed in the oscillator. The digital modulation signal is modulated on the carrier with the desired frequency deviation in this fashion.
The oscillator may be realized, for example, in the form of a crystal oscillator or an LC-oscillator. In a crystal oscillator, the oscillation frequency is primarily determined by the oscillation frequency of the crystal. In an LC-oscillator, however, at least one inductor and at least one capacitor influence the oscillation frequency.
For example, in order to achieve a frequency deviation of 60 KHz, it is necessary to connect and disconnect comparatively large capacitors in the so-called oscillator tank. However, the connecting and disconnecting of capacitors is always associated with charge injection problems. Undesirable interferences not only occur in the power supply of the oscillator, but also in the substrate terminal of an integrated oscillator. These charges negatively affect the oscillator, for example, a voltage-controlled oscillator, and also influence its output spectrum in an interfering fashion. In addition, the integration of a reconnectable capacitor requires a relatively large chip surface.